Ion acceleration using synchrocyclotrons is a mature technology that is well suited to produce high energy, but relatively low average ion beam currents. Acceleration is achieved by applying high frequency (typically radio frequency (RF)) electric fields to an ion beam packet as it spirals outward from the center of an axisymmetric, static magnetic field. It is well known that the frequency of the RF drive in synchrocyclotrons needs to be adjusted as the ion beam is being accelerated. The RF drive can be extended to include the variable frequency RF generator, RF power amplifier or amplifiers, and a structure or structures inside the magnetic field (such as RF cavities or dees) where the acceleration electric field is applied to the ion beam packet. Because the RF frequency varies during acceleration, typically there is only one bunch of ions in the device at any one time. The cyclotron frequency varies to compensate for changes to the relativistic mass of the accelerated particles as their energy increases during acceleration and the fact that the magnetic field is varying radially in order to provide beam focusing. The magnetic field in the bore of the machine needs to satisfy the following requirements for orbit stability. The value of the magnetic field needs to decrease with increasing radius, while keeping the value of0<2νz<0.5νr whereνz=n1/2,νr=(1−n)1/2,andn=−d log(B)/d log(r)over the accelerating region, and it needs to rise quickly with radius in the extraction region.
A body of literature exists on the control of the frequency of the RF acceleration. The object of the prior art has been to adjust the RF frequency to match the cyclotron frequency of the ion beam, while monitoring changes to the beam current after extraction. In addition, another object of the prior art has been to match a resonant circuit and the RF drive that it generates to the required frequency. No effort has been made to either monitor the phase of the ion beam orbits relative to the phase of the RF drive, or to adjust the phase and amplitude of the RF drive and the ion beam during injection, acceleration or extraction. In this case, the amplitude of the RF drive actually refers to the magnitude of the acceleration electric field applied to the beam by the RF structures. It is well known that if the relative phase between the ion beam orbit and the RF drive results in a substantial phase difference, the RF drive does not increase the beam energy, but instead decreases the energy of the ion beam by extracting energy from it. The ion beam continues to lose energy until it has drifted enough in phase and frequency to again match that of the RF drive: as the particles are decelerating, they are moving into regions of increasing magnetic field (at smaller radii) that require increased frequency for synchronism, but the applied RF field is decreasing in frequency, so the particles eventually slow down enough to the point where they are again in phase with the RF field and resume acceleration. Although eventually the beam packet gets accelerated, the beam quality suffers and the average beam current decreases. It would be best if the phase of the RF drive and the phase of the beam orbits were synchronized throughout the injection, acceleration and extraction process, especially for conditions where the final beam energy is varied (by adjusting the current in the cyclotron coils). For operation, the currents in all the coils in the cyclotron are varied by the same ratio which is adjusted in order to vary the final energy of the beam. It is usually that only about 50% of the electric field from the RF drive is accessible for beam acceleration in a conventional machine.
For synchrocyclotrons that use significant quantities of iron to generate and shape the acceleration field, changes to the coil currents (for example, to change the beam energy) change not only the intensity of the magnetic field, but also the magnetic field profile. Thus, an iron containing cyclotrons is not suitable for producing beams where the extracted beam energy can be varied, without the use of energy degraders or internal targets (for adjusting the charge of the ions).
In synchrocyclotrons, the beam orbits are controlled by the RF drive. This is the case when the frequency of the RF drive varies slowly. When the frequency of the RF increases rapidly (for example, when larger average currents are desired), the beam can lose synchronization with the RF energy, with results being very small acceleration or no current at all. In addition, it would be beneficial to control the RF phase and amplitude during both the injection of the ion beam as well as during the extraction. Injection control can be adjusted externally by pre-bunching the beam, so that it matches the acceptance angle of the cyclotron accelerating field. Control of the pre-buncher would, of course, be coordinated with the phase of the RF drive applied during the initial beam orbits of the acceleration cycle. However, for extraction, the opportunities are very limited. Adjustment of ion energy, phase and location of the ion beam during the last few orbits prior to extraction would allow better extraction efficiencies and minimize loss of beam that impacts radiation safety, heating and radiation damage to internal components. The ability to precisely control beam extraction in synchrocyclotrons is especially important for iron-free machines which can be designed to deliver output beams over a wide range of energies from a single machine without need for energy degraders in the output beam path (through the variation of the current in the cyclotron coils).
Therefore, it is a goal of the present disclosure to be able to directly vary the final energy of the beam extracted from a single cyclotron. A further objective is to maintain a high extraction efficiency regardless of the final beam energy. The variable energy is facilitated by the variation of the current in the cyclotron coils and adjustment of the main fields in the cyclotron. The final beam energy is a function of the magnitude of the magnetic field in the cyclotron.